This invention relates to a method of producing a Cu(In,Ga)(Se,S)2 semiconductor film, and a semiconductor film obtained by such a method. The present invention is also directed to a photovoltaic device having the above semiconductor film.
Thin-films of CuInSe2 (CIS), CuGaSe2 (CGS), Cu(In1-xGax)Se2 (CIGS) and Cu(In1-xGax)(SeyS1-y)2 (CIGSS), all of which are generically referred to as Cu(In,Ga)(Se,S)2, have been attracted much attention as a light absorbing film for use in photovoltaic devices or solar cells in recent years. While CIS which is a direct transition semiconductor has much greater absorption efficiency as compared with Si-type indirect transition semiconductors, the bandgap is relatively small. CIGS, in which part of indium is substituted by gallium, or CIGSS in which part of selenium of CIGS is substituted by sulfur, exhibit increased bandgaps.
In order to produce solar cells having uniform characteristics, it is necessary to control the composition of the multicomponent elements. However, because of resemblance in phase diagram of multicomponent compounds, it is very difficult to obtain desired Cu(In,Ga)(Se,S)2 with reproducibile quality from run to run. The following methods have been adopted to obtain Cu(In,Ga)(Se,S)2 films.
Namely, a method is known in which Cu is used in a stoichiometrically excess amount. This method has a problem because a separate phase of a Cuxe2x80x94Se compound is additionally formed.
A method in which (In,Ga) is used in excess is also known. While this method can form a single phase product, Cu/(In,Ga) is not 1:1, so that satisfactory conversion efficiency is not obtainable.
A method is further known in which Cu is first fed in an excess amount and (In,Ga) is then fed in an excess amount. While this method is superior to the above two methods, it is still difficult to obtain Cu(In,Ga)(Se,S)2 films with consistent compositions.
Accordingly, it is a general object of this invention to provide a method which can produce a better quality Cu(In,Ga)(Se,S)2 thin film more consistently and more predictably than known methods.
In accordance with the present invention, there is provided a method of producing a semiconductor film of Cu(MIII)(MVI)2 wherein MIII represents In1-xGax where x is between 0 and 1 and MVI represents SeyS1-y where y is between 0.5 and 1, comprising the steps of:
(a) depositing on a substrate a precursor Cu(MIII)(MVI)2 film having a molar ratio of Cu:MIII of less than 1.0:1.0 but not less than 1.0:1.4 and
(b) annealing said precursor film at a temperature of 400-500xc2x0 C. in an oxygen-containing atmosphere to form a buffer layer of indium oxide and/or gallium oxide and a Cu(In1-xGax)(SeyS1-y)2 film interposed between said substrate and said buffer layer.
The buffer layer may be removed to leave the Cu(In1-xGax)(SeyS1-y)2 film on the substrate.
The present invention also provides a semiconductor film of Cu(In1-xGax)(SeyS1-y)2 obtained by the above method.
The present invention further provides a photovoltaic device having the above semiconductor film.